The invention relates to a process for surface treatment of substrates composed of thermoplastic plastics. The invention also includes a device for working the method.
In film manufacture, the conventional state of the art includes subjecting film surfaces to an electrical corona treatment to increase the bonding strength or to achieve improved printability. The function of this process provides for guiding the film to be treated over an electrically grounded support surface, which in most cases is a rotatable roller, as well as subjecting the side of the film facing away from the supporting surface to an electrical corona discharge, generated by supplying an electrode, located at a distance from the support surface, with a high frequency, high-voltage alternating current. Over the years, a wide variety of electrode designs have been developed as for example plate, wire, comb, knife, half-dish, spring, or spindle electrodes made of a wide variety of materials as for example metals, metal oxides, or, increasingly in recent times, oxide ceramics, which however change nothing as far as the principle of the procedure is concerned.
Usually, surface treatment is performed under atmospheric pressure and in the presence of air. The search for a way of increasing pretreatment effectiveness has led to the development of methods using substances capable of reacting chemically. Such methods are described for example in U.S. Pat. No. 3,142,630, British Pat. No. 938,325 and U.S. Pat. No. 3,274,089 among others.
The characterizing feature of all these technologies is blowing gases capable of reacting into the corona discharge ignited between the two electrodes.
An expanded embodiment of this method is the so-called plasma processing, which is charactrized by the fact that the process is conducted in a vacuum to increase the kinetic energy of the ions and electrons in the electrical field. U.S. Pat. No. 3,057,792 is cited as representative of this type of processing.
As is evident from the description of the state of the art, only gases are generally used as the reactive media. Consequently, an object of the present invention was to develop a process which also allows using reactive substances in liquid form, thus considerably expanding the possibilities of surface modification.